1. Field of the Invention
The present invention relates to a video encoding apparatus, a video encoding method, a video encoding program, a video decoding apparatus, a video decoding method and a video decoding program.
2. Related Background of the Invention
Generally, in a video encoding apparatus, a coding target frame is divided into a plurality of blocks of predetermined size, and motion compensation prediction between each of the blocks and a prediction reference image of a predetermined region in a reference frame is performed so that motion vectors are detected, thus producing a predicted image of the coding target frame. In the video encoding apparatus, the coding target frame is expressed by motion vectors from the reference frame, so that the redundancy existing in the time direction is reduced. Furthermore, a prediction residual image based on a difference between the coding target frame and the predicted image is converted by DCT (Discrete Cosine Transform), and is expressed as a set of DCT coefficients, so that the redundancy existing in the spatial direction is reduced.
In the abovementioned video encoding apparatus, in order to achieve a further reduction of the redundancy existing in the time direction, the motion compensation prediction is performed with a high resolution by disposing interpolated pixels at the 1/2 pixel positions or 1/4 pixel positions between the integer pixels of the reference frame, so that the encoding efficiency is improved. A pixel value obtained by applying linear filter of (1, −5, 20, 20, −5, 1)/16 to 6 integer pixels that include 3 neighborhood integer pixels each on the left and right is given to the interpolated pixel that is located in the 1/2 pixel position between the integer pixels that are lined up in the horizontal direction. A pixel value obtained by applying a linear filter of (1, −5, 20, 20, −5, 1)/16 to 6 integer pixels that include 3 neighborhood integer pixels each above and below is given to the interpolated pixel that is located in the 1/2 pixel positions between the integer pixels that are lined up in the vertical direction. A mean value of the pixel values of interpolated pixels in the 1/2 pixel positions which are adjacent in the horizontal direction is given to the interpolated pixel that is located at equal distances from four neighborhood integer pixels. Furthermore, a linearly interpolated value from two pixels among the neighborhood integer pixels or interpolated neighborhood pixels in the 1/2 pixel positions is given to the interpolated pixel that is in the 1/4 pixel position. Namely, pixel values obtained by applying filtering to neighborhood integer pixels are given to the interpolated pixels, so that even in cases where the difference between the reference frame and the coding target frame is large. Thus the redundancy is effectively reduced.
Here, a video encoding apparatus is known in which motion compensation prediction is performed by giving the means values of four neighborhood integer pixels to the pixels at the (3/4, 3/4) pixel positions in order to improve the filtering effect further (for example, see G. Bjontegaard, “Clarification of “Funny Position””, ITU-T SG 16/Q15, doc. Q15-K-27, Portland, 2000.). In such a video encoding apparatus, the interpolated pixels are provided by using low-pass filters of which spectral band-pass in low frequency band is narrower than filter corresponding to linear interpolation, thereby improving the effect of filtering further. As a result, the redundancy is reduced. The interpolated pixels to which low-pass filters of which spectral band-pass in low frequency band is narrow are applied are called “Funny Positions”.